1. Field of the Invention
The present invention relates to an optical scanning device and an image forming apparatus using the same. In particular, the present invention relates to an optical scanning device suitable to an image forming apparatus having, for example, an electrophotographic process such as a laser beam printer, a digital copying machine, or a multifunction printer. The image forming apparatus is structured such that a plurality of light fluxes (light beams) emitted from a light source means including a plurality of light emitting sections are deflected by a polygon mirror as an optical deflector and then a surface to be scanned (hereinafter referred to as object surface) is scanned with the deflected light fluxes through a scanning lens system having an fθ characteristic to record image information.
2. Related Background Art
Up to now, in an optical scanning device used for an image forming apparatus such as a laser beam printer, a digital copying machine, or a multifunction printer, a light flux emitted from a light source means is guided to an optical deflector by an incident optical system. The light flux deflected by the optical deflector is imaged in a spot shape onto a photosensitive drum surface as an object surface by a scanning lens system, so that the photosensitive drum surface is scanned with the light flux.
In such an optical scanning device, in order to accurately control a write start position of an image on the photosensitive drum surface, a synchronous position detecting unit (BD optical system) as described below is generally provided in a position immediately before the write start of an image signal.
FIG. 6 is a main part sectional view of a conventional optical scanning device including a synchronous position detecting unit in a main scanning direction (main scanning sectional view). FIGS. 7A and 7B are each a main part sectional view when only an optical path of a light flux traveling to the synchronous position detecting unit in FIG. 6 is developed. FIG. 7A is a main scanning sectional view and FIG. 7B is a sub-scanning sectional view. In FIGS. 7A and 7B, a solid line indicates a light flux.
In FIG. 6, a light source means 81 is composed of, for example, a semiconductor laser. An aperture stop 82 forms a light flux emitted from the light source means 81 in a desired optimum beam shape. A collimator lens 83 changes the light flux passing through the aperture stop 82 into a substantially parallel light flux (divergent light flux or convergent light flux). A cylindrical lens 84 has a predetermined refracting power only in the sub-scanning direction. Note that each of elements such as the aperture stop 82, the collimator lens 83, and the cylindrical lens 84 corresponds to an element of an incident optical system 93.
An optical deflector 85 is composed of, for example, a rotating polygonal mirror (polygon mirror) and rotated in a direction indicated by an arrow A in FIG. 6 at a constant speed by a drive unit such as a motor (not shown). A single fθ lens (scanning lens) 91 with an fθ characteristic has a tilt correction function in which a conjugate relationship is made between the vicinity of a deflection surface 85a of the optical deflector 85 and the vicinity of a photosensitive drum surface 92 as an object surface within the sub-scanning section.
An imaging lens for synchronous detection (hereinafter referred to as BD lens) 86 images a synchronous signal detecting light flux (BD light flux) for adjusting timing of a scanning start position on the photosensitive drum surface 92 onto the surface of a BD slit 88 described later within the main scanning section. In addition, the imaging lens has a so-called surface tilt correction function in which the tilt of the deflection surface of the optical deflector 85 is corrected within the sub-scanning section.
A reflecting mirror for synchronous detection (hereinafter referred to as BD mirror) 87 reflects a BD light flux to a synchronous detection element 90 described later. A slit for synchronous detection (hereinafter referred to as BD slit) 88 is disposed in a position equivalent to the photosensitive drum surface 92.
A conjugate lens for synchronous detection (hereinafter referred to as BD conjugate lens) 89 is composed of a co-axial lens (spherical lens) in which a curvature in the main scanning direction is equal to that in the sub-scanning direction. In order to correct the surface tilt of the BD mirror 87 within the sub-scanning section, the conjugate lens is disposed such that the surface of the synchronous detection element 90 and the surface of the BD mirror 87 become a substantially conjugate relationship within the main scanning section and the sub-scanning section.
The synchronous detection element 90 is an optical sensor (hereinafter referred to as BD sensor). In FIG. 6, timing of the scanning start position for image recording on the photosensitive drum surface 92 is adjusted based on a synchronous signal (BD signal) obtained by detecting an output signal from the BD sensor 90.
Note that each of elements such as the BD lens 86, the BD mirror 87, the BD slit 88, the BD conjugate lens 89, and the BD sensor 90 corresponds to an element of the synchronous position detecting unit (BD optical system).
As shown in FIG. 7B, the above-mentioned BD lens 86 is set such that a conjugate point with the BD slit (BD slit surface) 88 is located on the deflection surface 85a within the sub-scanning section. Therefore, for example, when a multi-beam laser having a plurality of light emitting points is used as a light source means, heights of respective BD light fluxes crossing the BD slit 88 in the sub-scanning direction are different from one another. Accordingly, a time interval of the respective BD light fluxes reaching the BD sensor 90 is different from a design value by an influence of an error of straight travel property resulting from a manufacturing error of the BD slit 88, so that a variation in write start position (write start jitter) on the photosensitive drum surface 92 is caused.
Also, when it is structured such that a conjugate point with the BD slit 88 is located in the aperture stop 82 for determining a beam diameter on the photosensitive drum surface 92, the heights of the respective BD light fluxes crossing the BD slit 88 in the sub-scanning direction are equal to one another. Therefore, the influence of the error of straight travel property resulting from the manufacturing error of the BD slit 88 can be reduced. However, the spot diameter on the surface of the BD sensor 90 becomes too large, so that the amount of BD light flux incident to the surface of the BD sensor 90 becomes smaller. Accordingly, the following problem is caused. In other words, a response characteristic of the BD sensor 90 is deteriorated, so that a variation in the amount of respective BD light flux is easy to detect, thereby causing a jitter.